Modulating refrigerating apparatus



May 5, 1953 D. F. ALEXANDER I 2,537,175

MODULATING REFRIGERATING APPARATUS Original Filed Nov. 20, 1945 5 Sheets-Sheet 1 I6 70 HH HH 40 2 f s m L\\\\\\\\\\ \\\l FIG. I

5, 1953 D. F. ALEXANDER 2,637,175

MODULATING REFRIGERATING APPARATUS Original Filed Nov. 20, 1945 5 Sheets-Sheet 2 \J 62 i MJM, INVENTOR.

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llllllllll 'I'IIIY'I" May 5, 1953 D. F. ALEXANDER 5 1 75 MODULATING REFRIGERATING APPARATUS Original Filed Nov. 20, 1945 5 Sheets-Sheet 4 lllllllAlll IIIIIIIIIII SD 3- M5. 'INVENTOR. {1- 5] BY %I' M*% May 5, 1953 D. F'. ALEXANDER MODULATING REFRIGERATING APPARATUS Original Filed Nov. 20, 1945 5 Sheets-Sheet 5 124M Alllll lIl l fl lllllllllll "'I'II'I" M M I INVENTOR.

BY W M i Patented May 5, 1953 UNITED STATES PATENT OFFICE MODULATING REFRIGERATING APPARATUS Donald F. Alexander, Oakwood, Ohio, assignor to General Motors Corporation, Dayton, Ohio, a corporation of Delaware Continuation of application Serial No. 629,757,

November 20-, 1945. This application Decembe! 16, 1949, Serial No. 133,366

15 Claims. (Cl. 62-4) 1 This application is a continuation, of :my co pending application Serial No. 629,757 filed November 20, 1945, which has become abandoned since the filing of this application.

are energized when the generator is in operation and only a small amount of refrigeration is required;

Fig. 4 is a view similar to Figs. 2 and 3 showing This invention relates to refrigerating appara- 5 the circuit when the generator is in operation tus and more particularly to an improved air and full refrigeration'is required; and conditioning system suitable for use on a railway Fig. 5 shows the circuit when the generator is car, not in operation and the thermostats call for It is an object of this invention. to provide immaximum refrigeration available. proved controls for the refrigerating appara tus For purposes of illustration, I have shown my and the electrical equipment. invention applied to a conventional railway car Another object of this invention is to provide It whereas some of the features of my invention for a reduction in refrigeration output in reare equally applicable to other types of installasponse to either a reduction in refrigeration r tions. quirements or a reduction in the power available The refrigerating system used for cooling the for operating the compressor motor. air comprises a pair of evaporator coils l2 and Another object of this invention is to provide 14 which are preferably arranged in the upper apparatus which is economical andtrouble-free end zone of a car as shown in Fig. 1. A blower operation. a unit [6 circulates air to be conditioned over the A. still further object of this invention is to coils l2 and M. The evaporator coils and the provide means for reducing the electrical load blower unit It are disposed within an enclosure when the main generator is not in operation or 10 which is provided with a return air inlet is operating at a low speed. and an air outlet 22 arranged as shown. Ref- Still another object of this invention is to proerence numeral 24 designates a conventional revide a control system for the compressor motor ri rant co pr Whichis Operated b re which makes it possible to start, the. motor with current motor 25 which drives the compressor maximum field excitation regardless of the re- 24 through the belt 30. Compressed refrigerant frigeration requirements. leaves the compressor 24 through the line 32 Reference is made to the following applicawhich leads to a conventional condenser 34 over tions and. patents owned by the same assignee: :1; which air is circulated by the fan unit 36. The N. 616,078, filed September 13, 1945, Donald F. refrigerant condensed by the condenser 34 flows Alexander, applicant, Pat. 2,536,248, granted Janinto the receiver 31 from whence the liquid reuary 2, 1951; S- N. 652,514, filed March 7, 1946, frigerant is supplied to the evaporator coils l2 Donald Alexander and Calvin J. Werner, apand [4 through the liquid line 38. The complicants, Pat. 2,541,904, granted February 13, 1 pressor and condenser mechanism is preferably 1951; S. N. 652,515, filed March 7, 1946, Donald F. mounted beneath the car so as not to require the Alexander, applicant, Pat; 2,608,067, granted use of valuable passenger space.

August 26, 1952 A thermostatic expansion valve controls the Further objects and advantages of the present flow of liquid refrigerant into the evaporator invention will be apparent from the following section I 4 in accordance with well-known pracdescription, reference being had to the accomtice. The expansion valve 40 is provided with panyi-ng' drawings, wherein: a preferred form of the usual thermostatic bulb 42 arranged in the present inventionis clearly shown. thermal exchange with the outlet line leading I th drawings; from the evaporator section [4 as shown. The

Fig. 1 is a diagrammatic view showing a raii- 45 flow of refrigerant to the evaporator section 12 way car provided with my improved air condiis controlled by a solenoid valve 44 and a contioning system ventional thermostatic expansion valve 46 which Fig. 2 is a diagrammatic view of the electrical are arranged in series. The thermostatic exclrouits showing in heavy linesv those portions of pansion valve 46 is provided with the usual form the circuit which are energized when the genof thermostatic bulb 48 arranged in thermal exerator is in operation but no refrigerationisrechange with the outlet line leading from the quireel and showing in light lines those portions evaporator I2. of the circuit which are deenergized; As will be explained more fully hereinafter,

Fig. 3 is a view similar to Fig. 2 showing In both of the evaporator sections aresupplied with heavy lines those portions of the circuit which liquid refrigerant when maximum refrigeration is required but as the requirement for refrigeration decreases the solenoid valve 44 is closed with the result that only the surface of the evaporator section I4 is effective for cooling the a 1r for the enclosure. By virtue of this reduction in the effective evaporator surface, the need for frequent cycling of the compressor is eliminated and a more uniform temperature and relator G and the motor 54 are preferably combined into a unit having a single frame reduiring only one shaft and oneset of bearings for the combined unit. A clutch 55 is provided between the unit and the axle 58 of the car III so that the motor 54 may be used for operating the enerator 50 when the railway car I ii is standin still in the station. The motor 54 is preferably a three-phase alternating current motor which may be plugged in at any of the railway terminals pro ided with three-phase power. The enerator 50 is a direct current enerator which serves to charge the storage batteries 52 and to supply electrical energy to the electrical equipment on the car.

Referrin now to the circuit diagram shown in Figs. 2 through 5. reference numerals M and 6| designate themain powerlines which are arranged to receive power from either the battery 52 or the generator 50. ,-A conventional reverse current relay 62 is rovided in the lines leading from the generator 58 to the main power lines 39 and BI as shown.

A manually o erated master control switch M is provided in the line 65 for turnin on and off the entire air conditionin vsvstem. The e aporator blower It is connected directly across the power lines and operates continuously as lon as the switch 6 remains closed w ereas the operation of the com ressor motor 25 is further c ntrolled by a manually o erated switch M which.

in turn places the compressor motor 26 under control of the relay 62 and the thermostats IE3 and I2. The thermostats Ill and I2 are preferably located so as to respond to the temperature within the conditioned space. The thermostat Ii! is preferably set to remain closed at tem eratures above 74 F. whereas the thermostat i2 is preferably set to remain closed at tem eratures above 73 F. It is to be understood that these temperature values are given primarily for purposes of illustrations since the thermostats could be set to operate at other temperature values.

The thermostat I2 which is the first to close as the temperature becomes, high enough to require refrigeration controls the starting and stopping of the compressor motor in a manner to be described hereinafter whereas the thermostat H1 is arranged in series circuit relationship with a solenoid I4 which serves to control the switches I6 and 18 which in turn reduce the speed of the motor and shut off the flow of refrigerant to the evaporator section I2 when the need for refrigeration decreases. The switch it is arranged in series with the solenoid valve 44 so that when the need for refrigeration again increases and the thermostat It closes the circuit through the sole- 4 noid I4, the switch 16 will close the circuit to the solenoid valve 44 thereby allowing the flow of refrigerant to the evaporator section I2. The switch 78 is arranged in series with the solenoid 80. which in turn controls .the compressor motor field resistance 83 through operation of switch 82. The arrangement is such that the compressor motor is caused to operate at a reduced speed whenever the thermostat it is open and the low voltage switch HIE.

motor does not fall below a predetermined value.

A second low voltage coil I98 is arranged in circuit as shown and also exerts a control on the A conventional low voltage reset switch II is provided as shown. Opening of the main switch 55 serves to deenergize the low voltage coils I24 and N8 with the result that prior to the closing of switch 65 the low voltage switchjItt will be in the down position, but just as soon as the switches 3 $8, and the low voltage reset switch I! are closed -the low voltage coil I93 will be energized, and if the voltage is high enough for proper operation of the equipment the switch I86 will be moved from its down position to itsup position, thereby energizing the solenoid I III which in turn closes the switches H2, H4, and IIE. Closing of the switch H4 causes the 'low voltage coil N38 to remain energized even after the manually operated reset switch II .has been reopened. Closing of the switch II6 is necessary before the thermostat :72 is capable of initiating operation of the compressor motor 26 as will be explained more fullyhereinafter.

When refrigeration is first needed after a period of shutdownf the thermostat 72 closes whereby current will flow from the power line 6| through the line 92, the thermostat 12, switch H6, the starting solenoid I I8 and the switch I20. Energization of the starting solenoid H8 first causes the switches I22 and I2I to close. Energization of the solenoid I It also serves to operate the delayed action switch I25 but'the switch I26 is provided with a spring connection I36] and a dash-pot mechanism I32 with the result that the compressor motor is given a chance to at least partially come up to speed befcrethe switch I25 closes.

Closing of the switch I28 causes energization of the solenoid I36 which in turn closes the switches I36 and I3? and opens the switch I20. 'The switch I29 is arranged in series with the solenoid MS with the result that the solenoid H8 becomes deenergized when the switch I20 opens and deenergization of solenoid H8 causes opening of the switches I22, I24, and I26. Before the solenoid IIS becomes thus deenergized, current will begin flowing from the power line 6| through the line I02, switch I24, armature resistance 90 and the armature of the motor 2t. A portion of the current flowing through the switch I24 will fiow through the line I38, switch I22, and the field coil MB of the compressor motor 26. By virtue of this circuit arrangement,

it is obvious that maximum field excitation is provided by the motor during the starting period, and it is also obvious that the armature resistonce 5,0 is arranged in series with the armature of the. motor so as to out down the armature current during the starting period, but just as soon as the delayed action switch 126 closes so as to energize the solenoid 134, the solenoid 1H8 becomes deenergized and the switches 122 and I24 together with switch I 26 itself are opened.

With switches 13S and it! closed and switches 42.2 and .124 open (see Fig. 3), the armature current flows directly to the armature through the closed switch .131 but the field current is required to flow, via the switch I37, through the armature resistance 9!), line 2, switch 82, and that portion of the field resistance 38 shown energized in Fig. '3. It will be noted that during the initial starting period when the switch I24 is momentarily closed, the condenser fan motor 35 is supplied with current via the switch I24 and armature resistance '50. It will also be observed that upon closing; of the switch I 31 the condenser fan motor 35 is connected directly across the power lines by means of the conductors I44 and :02. Those portions of the circuit which are energized whenrthe generator is in operation and the thermostat 2 is closed but the thermostat It is open have been shown in heavy lines in Fig. 3 of the drawing.

As the temperature within the conditioned space increases from 73 to 74 the thermostat 18 will close with the result that the circuit connecmotor thereby causing the compressor motor to operate at full speed. Closing of the switch Hi causes energization of the magnetic valve 44 with the result that refrigerant will be supplied to both sections of the evaporator and full re- Irigeration capacity will be available so long as the thermostat remains closed.

In the event that thegenerator 50 does not operate at a speed high enough to effectively generate current. the reverse current relay 62 will function and will cause the switch I46 to close, thereby short-circuiting a portion of the field remstance 8%. This in turn causes the motor 26 to operate at a slower speed and to reduce the power required. It is recognized that operation of the compressor motor at the slower speed will tend to reduce the capacity of the refrigeration; system due to the decrease in the compressor capacity. This reduction in capacity takes place irrespective of the temperature-within the conditioned space but takes place only when the generator is not in operation such as when the train is traveling slowly or is standing at the station without any standby motor operation. The standby motor 54 may be used at the stations for operating the generator so as to provide full generator capacity and full refrigeration when necessary.

Rsum of operation The air conditioning apparatus may be set into operation by closing the manually operated switches 54, 8,8 and H. Closing of the manually operated switch 64 will cause the blower motor to to circulate air within the vehicle irrespective of temperature conditions existing in the vehicle. Closing of the switch 68 and the reset '6 switch H will place the compressor motor. under control of the thermostats 12 and 10 which are set to remain closed at temperatures above 73 and 14 respectively.

Assuming that the temperature conditions within the space to be conditioned cause closing of the thermostat '12, the compressor motor 26 will be placed in operation. When the thermostat 12 first closes, the field resistance 38 will be short-circuited by the switch I22, and the armature resistance will be placed in series with the compressor motor armature but after the motor has had a chance to get started the armature resistance .90 will be cut out of the armature circuit by closing of the switch I31 and opening of the switch I24 with the result that the armature resistance 90 will be placed in series with any portion of the field resistance 88 which may be connected in series with the motor field MEL. When the thermostat 12 only is closed and the thermostat ill is open, the compressor motor will be operated with a strong field with the result that the motor will operate at a reduced speed. Under these same conditions only one of the evaporator sections is effective for cooling the air.

As the temperature within the conditioned space increases enough to cause closing of the thermostat 2'0, the solenoid valve M- will be energized with the result that both sections of the evaporator will be in. operation. Closing of the thermostat it also causes opening of the switch 82 with the result of an introduction of more fieldresistance which in turn causes an increase in the speed of the compressor motor 26- In theevent that the generator is ineffective, the switch M6 will open and the speed of the motor 25 will be partially reduced so as to conserve on the power required during; battery operation.

It will be noted that if the generator 56 discontinues operating when the thermostat i0 is in its open position thereby calling for only partial refrigeration capacity, operation of the switch I46 will have no effect on the compressor motor speed since the compressor motor speed will be reduced sufficiently to avoid execs-- sive battery discharge rate. By virtue of this arrangement it is apparent that the system eoonomices on the battery whenever necessary. It is also apparent that the compressor motor is never permitted to be brought up to speed immediately since the circuit provides for maximum field excitation during the starting period irrc spective of the position. of the thermostats.

While no means has been shown for varying the speed of the fan motors when the compressor motor speed is changed, it is obvious that the speed control arrangement used for the cornpressor motor could extended to control the ion motors. For example, the soienoid so could operate a speed varying resistance (not shown) in one or more of the fan motor circuits.

While the form of embodiment of the invention herein disclosed constitutes a preferred form, it is to be understood that other forms might be adopted, may come within the scope of the, claims which follow.

What is claimed as follows:

1. A single enclosure, e2 apparatus for cooling said enclosure including reirigerant iiquclying and evaporating means connected. an operative refrigerant circuit, said evaporatinc means being iocated in heat exchange relae tion with said enclosure, a motor for driving'said refrigerant liquefying "means; said motor. being provided with a speed regulating means, a first thermostatic means responsive to refrigerating requirements of said enclosure for initiating the operation of said motor, said thermostatic means including a starting control means for controlling said speed regulating means to. provide a limited motor starting speed for a given length of time and'an additional control means for controlling said speed regulating means to provide a higher motor speed for operation after the given length of time, and a second thermostatic means responsive to greater refrigerating requirements of said enclosure than said first thermostatic means for controlling said speed regulating means to provide a still higher motor speed for operation under greater refrigeration requirements.

2. A single enclosure, refrigerating apparatus for cooling said enclosure including refrigerant liquefying and evaporating means connected in an operative refrigerant circuit, said evaporating means being located in heat exchange relation with said enclosure, a single motor for driving said refrigerant liquefying means, said motor being provided with a speed regulating means, I

a first thermostatic means responsive to moderate refrigerating requirements of said enclosure for controlling said speed regulating means to provide a moderate motor speed for the moderate requirements, and a second cycling thermostatic means responsive to greater refrigerating requirements of said enclosure to control said speed regulating means to provide an increased motor speed whenever and only as long as the greater refrigerating requirements continue above a certain value and the power supply is ample, said second cycling control means being ineffective below said certain value, said evaporating means including a normally effective poi tion and a second portion, both portions being located in heat exchange relation with said enclosure, and a control means for preventing evaporation in said second portion, said second cycling thermostatic means including means operable coincidentally with the change in said speed regulating means for rendering ineffective said control means so as to permit evaporation in said second portion.

3. Refrigerating apparatus including refrigerant liquefying and evaporating means connected in an operative refrigerant circuit, a direct current electric motor for driving said refrigerant liquefying means, said motor including a field circuit and an armature circuit, a resistance means connected to said field circuit, control means responsive to refrigerating requirements for controlling said motor, said control means including means effective during the starting of the motor for transferring said resistance means from. said field circuit to said armature circuit for a controlled length of time and then returning said resistance means to the field circuit at the end of the controlled length of time.

4. Refrigerating apparatus including refrigerant liquefying and evaporating means connected in an operative refrigerant circuit, a direct current electric motor for driving said liquefying means, said motor including a field circuit, a resistance means of sufficient value to cause the motor to operate at the maximum desired speed, said resistance means being connected to the field circuit, a battery and a generating means for supplying current to the battery and to the motor, cycling means responsive whenever the refrigerating requirements, arebelow a certain value for reducing the effectivevalue of said-resistance means to increase the field strength in the motor for operation at reduced speed under reduced refrigerating requirements, and means responsive to a reduction below a certain value in therate ofgeneration by said generating means for reducing the effective value of the resistance means to'reduce the field strength and the current consumption and the discharge of the battery.

5. Refrigerating apparatus comprising in combination, an evaporator having a plurality of sections, means for flowing air in thermal exchange with said evaporator, means including a refrigerant compressor for supplying refrigerant to said evaporator, a multiple speed motor for operating said compressor, a generator for supplying energy to said motor, a battery for supplying energy to said motor when said generator is ineffective to operate said motor, means including a thermostat responsive to changes in refrigeration requirements in the conditioned space for rendering one of said sections of said evaporator inactive to cool-the air, and for causing said motor to operate at a reduced speed when the temperature within the conditioned space falls below'a predetermined value, and means responsive to energization of said motor solely from said battery for causing said motor to operate at a reduced speed irrespective of temperature conditions within said space.

6. Air conditioning apparatus comprising in combination, an evaporator having a plurality of independently controlled sections, means including a refrigerant compressor for supplying liquid refrigerant to said evaporator sections, a motor for operating said compressor comprising a field circuit, means for varying the resistance of said field circuit so as to vary the speedof said motor, means operable when the refrigeration requirements fall below 'a predetermined value for rendering one of said sections of said evaporator inactive and for reducing the speed of said motor by increasing the field excitation, switch means for starting and stopping said motor, and means operable in response to closing of said switch means for insuring maximum field excitation duringthe starting of said motor regardless of the refrigeration requirements.

'7. Air conditioning apparatus comprising in combination, an evaporator, a condenser, a compressor, refrigerant flow connections between said evaporator, compressor, and condenser, a motor for operating said compressor comprising a field circuit, means for varying the resistance of said field circuit so as to vary the'speed of said'motor, control means responsive to refrigeration require-' ments operable when the refrigeration requirements fall below a predetermined value for reducing the speed of said motor by increasing the field excitation thereof, switch means for starting and stopping said motor, and means operable in response to closing of said switch means for insuring maximum field excitation during the starting of said motor regardless of the refrigeration requirements.

8. In combination, an evaporator, a compressor, a condenser, refrigerant flow connections between said evaporator, condenser, and compressor, a motor for operating said compressor comprising a field circuit, means for varying the resistance of said field circuit so as to vary the speed of said motor, a first source of electrical energy for energizing said motor, a second relatively limited source of electrical energy for energizing said moto'nmeans operable when said motor 'is energized from the firstof said sources of electrical energy for decreasing the field excitation so as to operate said motor atmaximum speed, switch means for starting and stopping said motor, and means operable in response to closing of said switch means for insuringmaximum .field excitation during the starting of said motor regardless of which source of electrical energy is used for energizing said motor.

9. In combination with a vehicle JhaVinga live axle, a generator, torque transmitting means between said generator and said live axle, said torque transmitting means including a clutch for declutching said generator from said live axle, a standby motor for operating said generator when said live axle is ineifective to drive said generator, a first evaporator coil, a second evaporator coil, fan means for flowing air to be conditioned for said vehicle in thermal exchange with said evaporator coils, a condenser, a, compressor, refrigerant fiow connections between said compressor, condenser, and said evaporator coils whereby liquid refrigerant is supplied to said evaporator coils, a motor for operating said compressor, circuit means for supplying electrical energy from said generator to said compressor motor, a bat tery in said circuit means adapted to be charged by said generator and adapted to supply electrical energy to said compressor motor when said generator is not operating at generating speeds, first means responsive to refrigeration requirements for starting and stopping said compressor motor, second means responsive to refrigeration requirements for rendering one of said evaporator coils ineffective and for reducing the speed of said compressor motor when the refrigeration requirements fall below a predetermined value, and means responsive to the output of said generator for reducing the speed of said compressor motor irrespective of refrigeration requirements when the generator output falls below a given value.

10. Air conditioning apparatus comprising in combination, an evaporator, means for flowing air to be conditioned in thermal exchange with said evaporator, a compressor, a condenser, refrigerant fiow connections between said evaporator, condenser, and compressor, a motor for operating said compressor, a source of electrical energy, said compressor motor comprising a field circuit and an armature circuit, circuit connections between said motor and said source of electrical energy, said circuit connections comprising a first resistance means, means for connecting said first resistance means in series with the field circuit of said compressor motor, a second resistance means, means for selectively connecting said second resistance means either in series with said armature circuit or in series with said first named resistance and said field circuit, means responsive to refrigeration requirements for varying said first resistance means, and time delay means operable upon starting of said motor for short-circuiting said first resistance means for a predetermined length of time during the starting period of said motor.

11. A single enclosure, refrigerating apparatus for cooling said enclosure including refrigerant liquefying and evaporating means connected in an operative refrigerant circuit, said evaporating means being located in heat exchange relation with said enclosure, a single motor for driving said refrigerant liquefying means, said motor being provided with a two speed regulating means providing a moderate speed and full speed, a first 10 thermostatic means responsive to a definite moderate temperature of said enclosure for controlling saidspeed regulating means to provide said moderate motor speed for the moderate requirements, anda second cycling thermostatic means responsive to a definite higher temperature than said moderate temperature of said enclosure to control said speed regulating means to provide full :motor speed whenever and only .as long as the temperature of the enclosure is above said higher temperature and the power supply ,is amplarsaid second cycling control means being inefiective below said higher temperature.

'12. A single enclosure, refrigerating apparatus for cooling said enclosure :i'ncluding refrigerant liquefying and evaporating means connected in an operative refrigerant circuit, a direct current electric motor for driving said liquefying means, said motor including a field circuit, a resistance means connected in seri s with the field circuit, a first thermostatic means responsive to a moderate temperature of said enclosure for shunting a definite part of said resistance means in series with said field circuit to increase the field strength in said motor, and a second cycling thermostatic means responsive to a higher temperature than said moderate temperature of said enclosure for connecting all of said resistance means in series with said field circuit to reduce the field strength in said motor whenever and only as long as the greater refrigerating requirements continue above a certain value and the power supply is ample.

13. A single enclosure, refrigerating apparatus for cooling said enclosure including refrigerant liquefying and evaporating means connected in an operative refrigerant circuit, a direct current electric motor for driving said liquefying means, said motor including a field. circuit, a resistance means of sufficient value connected in series with the field circuit to cause the motor to operate at full speed, and cycling thermostatic means responsive whenever the temperature of said enclosure is below a certain value for shunting at least a part of said resistance means to increase the field strength in the motor to cause the motor to operate at a reduced speed under reduced refrigerating requirements, and a second thermostatic means responsive whenever the temperature of said enclosure is below a further reduced value 101' stopping the operation of said motor as long as the refrigerating requirements are below the reduced value.

14. rterrigerating apparatus including refrigerant liqueiying and evaporating means connected in an operative refrigerant circuit, means for preventing evaporation in one portion of said evaporating means, a direct current electric motor for driving said liquefying means, said motor including a field circuit, a resistance means of suhicient value to cause the motor to operate at full speed, said resistance means being connected to the field circuit, and cycling thermostatic means responsive whenever the refrigerating requirements are below a certain value for reducing the efiective value of said resistance means to increase the field strength in the motor to cause the motor to operate at a reduced speed under reduced refrigerating requirements, said cycling thermostatic means also having interlock means operable coincidentally with the reduction in effective value of said resistance means for effectively operating said means for preventing evaporation in one portion of the evaporating means 1 l While the remaining portion remains efiective for evaporation.

15. Refrigerating apparatus including refrig erant liquefying and evaporating means connected in an operative refrigerant circuit, a direct current electric motor for driving said refrigerant liquefying means, said motor including a field circuit, a resistance means connected to said field circuit, control means responsive to refrigerating requirements for controlling said motor, said control means including means effective during the starting period of the motor for decreasing the efiective value of said resistance means and time delay means interlocked with said means for decreasing said resistance means efiective after a given length of time for increasing the effective value of said resistance means at the end of said 

